Alan Keel, Criminalist at the San Francisco Police Department, San Francisco, California from 1996 to 1999 "revealed that there were two letters in possession of the department that, in contrast to the "true" Zodiac letters, had abundant saliva and DNA-containing oral epithelial cells on them, that DNA was easily extracted from these two letters, and that the DNA extracted from these two letters matched between them. These two letters were considered forgeries, since the "true" Zodiac verifiable letters had not been licked by the sender". Mike Rodelli, an avid Zodiac researcher, who conversed with Alan Keel, revealed "In contrast (to earlier communications), Keel analyzes two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both". Mike Rodelli.

​If you take a look at the San Francisco Police Department DNA report, the 1974 Exorcist letter is the only letter that year to produce any viable results, described as "cells found". The S.L.A letter was totally disregarded, and the "Citizen" and "Red Phantom" communications had no entry in comments. If Alan Keel is correct in his assertion, then the Exorcist letter would enter the classification of unlikely Zodiac correspondences - ones that had "been licked by the sender".

In the latest round of DNA testing in the Zodiac case, two of the three July 31st 1969 communications have again come under scrutiny. These were unavailable at the time the San Francisco Police Department DNA testing was undertaken. It is apparent that these letters have struggled to give up their secrets, as have other communications listed in the report, which includes the October 13th 1969 'Paul Stine' letter, the November 8th 1969 'Dripping Pen' card, the November 9th 1969 'Bus Bomb' letter, the December 20th 1969 'Melvin Belli' letter, the April 20th 1970 'My Name Is' letter, the April 28th 1970 'Dragon' card and the June 26th 1970 'Button' letter. In total, nine consecutive communications that, it is fair to say, have produced little in the way of a recognizable DNA fingerprint. Most of these are labelled in comments as "few cells".

The only remaining communications in the DNA report subsequent to June 26th 1970 which produced any notable results, classified as "cells found", were:[1] The July 24th 1970 'Kathleen Johns' letter.[2] The July 26th 1970 'Little List' letter, and[3] The January 29th 1974 'Exorcist' letter.And all three had one crucial element in common - The Mikado.

The 'Kathleen Johns' letter began the trilogy by stating "So now I have a little list, starting with that woeman + her baby that I gave a rather intersting ride for a coupple howers one evening a few months back that ended in my burning her car where I found them". This would be continued two days later with the paraphrasing of two of Gilbert and Sullivan's acts from The Mikado. The first section of the 'Little List' letter pulls lines from the A more humane Mikado, where the author uses the words billiard along with crooked cues and twisted shoes.This correspondence goes on to paraphrase Gilbert and Sullivan's As some day it may happen, performed by Ko-Ko, as did the future correspondence of the Exorcist Letter in 1974, when reciting Tit-Willowfrom Ko-Ko's On a tree by a river, part of Act Two.

We don't know the exact concentrations of DNA retrieved from these three communications, but it is evident that these were the only three letters classified as "cells found" - and all three made reference to The Mikado. If "cells found" could be proven as "saliva" found, and the "true Zodiac verifiable letters had not been licked by the sender" according to Alan Keel, then this could indicate that all The Mikado letters were not authored by the Zodiac Killer. An extremely hard notion to accept when we look at the handwriting and design of each of these correspondences.

One can understand why certain sections of the Zodiac communications were withheld from the public, such as his bomb diagrams - yet the entire July 26th 1970 'Little List' letter was not released to the newspapers until October 12th 1970. This innocuous correspondence for the most part was withheld for two and a half months until it featured in a San Francisco Chronicle article entitled 'Gilbert and Sullivan Clue to Zodiac',in tandem with the subsequent October 5th 1970 '13 Hole' postcard. If the 'Little List' letter and '13 Hole' postcard were authored by the Zodiac Killer, then it is notable that, despite putting a lot of effort into this rather lengthy correspondence on July 26th 1970, he failed to make any issue about the complete absence of newspaper coverage that this letter received when he mailed his next correspondence. It is believed he repeated his victim count of 13 because his last letter hadn't been published in the newspapers. But could it have been the case that the Zodiac Killer wasn't reaffirming the victim count of 13 because his previous correspondence wasn't published, rather, he was unaware somebody had mailed the 'Little List' letter in his name? Hence his lack of concern about "front page coverage", or for that matter, any coverage at all.

If the Zodiac Killer hadn't written any of The Mikado communications, then he also wasn't claiming the abduction of Kathleen Johns on March 22nd 1970. In fact, the notion of a killer driven by the theatrical librettos of Sir William Schwenck Gilbert would be quashed entirely. It is extremely difficult to sell a story of a Zodiac Killer who didn't author any of The Mikado communications, let alone all of the 1974 communications - so the curtain call will be cancelled for this performance only.